1. Field of the Invention
This invention relates to rotary compressors or blowers, particularly to blowers of the backflow type. More specifically, the present invention relates to reducing airborne noise associated with Roots-type blowers employed as superchargers for internal combustion engines.
2. Description of the Prior Art
Rotary blowers particularly Roots-type blowers are characterized by noisy operation. The blower noise may be roughly classified into two groups: solid borne noise caused by rotation of timing gears and rotor shaft bearings subjected to fluctuating loads, and fluid borne noise caused by fluid flow characteristics such as rapid changes in fluid velocity. Fluctuating fluid flow contributes to both solid and fluid borne noise.
As is well-known, Roots-type blowers are similar to gear-type pumps in that both employ toothed or lobed rotors meshingly disposed in transversely overlapping cylindrical chambers. Top lands of the lobes sealingly cooperate with the inner surfaces of the cylindrical chambers to trap and transfer volumes of fluid between adjacent lobes on each rotor. Roots-type blowers are used almost exclusively to pump or transfer volumes of compressible fluids, such as air, from an inlet receiver chamber to an outlet receiver chamber. Normally, the inlet chamber continuously communicates with an inlet port and the outlet chamber continuously communicates with an outlet port. The inlet and outlet ports often have a transverse width nominally equal to the transverse distance between the axes of the rotors. Hence, the cylindrical wall surfaces on either side of the ports are nominally 180.degree. in arc length. Each receiver chamber volume is defined by the inner boundary of the associated port, the meshing interface of the lobes, and sealing lines between the top lands of the lobes and cylindrical wall surfaces. The inlet receiver chamber expands and contracts between maximum and minimum volumes while the outlet receiver chamber contracts and expands between like minimum and maximum volumes. In most Roots-type blowers, transfer volumes are moved to the outlet receiver chamber without compression of the air therein by mechanical reduction of the transfer volume size. If outlet port air pressure is greater than the air pressure in the transfer volume, outlet port air rushes or backflows into the volumes as they become exposed to or merged into the outlet receiver chamber. Backflow continues until pressure equalization is reached. The amount of backflow air and rate of backflow are, of course, a function of pressure differential. Backflow into one transfer volume which ceases before backflow starts into the next transfer volume, or which varies in rate, is said to be cyclic and is a known major source of airborne noise.
Another major source of airborne noise is cyclic variations in volumetric displacement or nonuniform displacement of the blower. Nonuniform displacement is caused by cyclic variations in the rate of volume change of the receiver chamber due to meshing geometry of the lobes and due to trapped volumes between the meshing lobes. During each mesh of the lobes first and second trapped volumes are formed. The first trapped volumes contain outlet port or receiver chamber air which is abruptly removed from the outlet receiver chamber as the lobes move into mesh and abruptly returned or carried back to the inlet receiver chamber as the lobes move out of mesh. As the differential pressure between the receiver chambers increases, so does the mass of carry-over air to the inlet receiver chamber with corresponding increases in the rate of volume change in the receiver chambers and corresponding increases in airborne noise. Further, blower efficiency decreases as the mass of carry-over air increases.
The trapped volumes are further sources of airborne noise and inefficiency for both straight and helical lobed rotors. With straight lobed rotors, both the first and second trapped volumes are formed along the entire length of the lobes, whereas with helical lobes rotors, the trapped volumes are formed along only a portion of the length of the lobes with a resulting decrease in the degrading effects on noise and inefficiency. The first trapped volumes contain outlet port air and decrease in size from a maximum to a minimum, with a resulting compressing of the fluid therein. The second trapped volumes are substantially void of fluid and increase in size from a minimum to a maximum with a resulting vacuum tending expansion of fluid therein. The resulting compression of air in the first trapped volumes, which are subsequently expanded back into the inlet port, and expansion of the second trapped volumes are sources of airborne noise and inefficiencies.
Many prior art patents have addressed the problems of airborne noise. For example, it has long been known that nonuniform displacement, due to meshing geometry, is greater when rotor lobes are straight or parallel to the rotor axes and that substantially uniform displacement is provided when the rotor lobes are helically twisted. U.S. Pat. No. 2,014,932 to Hallett teaches substantially uniform displacement with a Roots-type blower having two rotors and three 60.degree. helical twist lobes per rotor. Theoretically, such helical lobes could or would provide uniform displacement were it not for cyclic backflow and trapped volumes. Nonuniform displacement, due to trapped volumes, is of little or no concern with respect to the Hallett blower since the lobe profiles therein inherently minimize the size of the trapped volumes. However, such lobe profiles, in combination with the the helical twist, can be difficult to accurately manufacture and accurately time with respect to each other when the blowers are assembled.
Hallett also addressed the backflow problem and proposed reducing the initial rate of backflow to reduce the instantaneous magnitude of the backflow pulses. This was done by a mismatched or rectangular shaped outlet port having two sides parallel to the rotor axes and, therefore, skewed relative to the traversing top lands of the helical lobes. U.S. Pat. No. 2,463,080 to Beier discloses a related backflow solution for a straight lobe blower by employing a triangular outlet port having two sides skewed relative to the rotor axes and, therefore, mismatched relative to the traversing lands of the straight lobes. The arrangement of Hallett and Beier slowed the initial rate of backflow into the transfer volume and therefore reduced the instantaneous magnitude of the backflow. However, neither teaches nor suggests controlling the rate of backflow so as to obtain a continuous and constant rate of backflow.
Several other prior art U.S. patents have also addressed the backflow problem by preflowing outlet port or receiver chamber air into the transfer volumes before the lands of the leading lobe of each transfer volume traverses the outer boundary of the outlet port. In some of these patents, preflow is provided by passages of fixed flow area through the cylindrical walls of the housing sealing cooperating with the top lands of the rotor lobes. Since the passages are of fixed flow area, the rate of preflow decreases with decreasing differential pressure. Hence, the rate of preflow is not constant.
U.S. Pat. No. 4,215,977 to Weatherston discloses preflow and purports to provide a Roots-type blower having uniform displacement. However, the lobes of Weatherston are straight and, therefore, believed incapable of providing uniform displacement due to meshing geometry.
The Weatherston blower provides preflow of outlet receiver chamber air to the transfer volumes via circumferentially disposed, arcuate channels or slots formed in the inner surfaces of the cylindrical walls which sealingly cooperate with the top lands of the rotor lobes. The top lands and channels cooperate to define orifices for directing outlet receiver chamber air into the transfer volumes. The arc or setback length of the channels determines the beginning of preflow. Weatherston suggests the use of additional channels of lesser setback length to hold the rate of preflow relatively constant as pressure in the transfer volumes increases. The Weatherston preflow arrangement, which is analogous to backflow, is believed theoretically capable of providing a relatively constant preflow rate for predetermined blower speeds and differential pressures. However, to obtain relatively constant preflow, several channels of different setback length would be necessary. Further, accurate and consistent forming of the several channels on the interior surface of the cylindrical walls is, at best, an added manufacturing cost.
The prior efforts of Hallett, Beier, and Weatherston have, in some cases, provided less than optimum reduction in airborne noise and, in some cases, reduced volumetric efficiency of the blowers. These disadvantages are greatly reduced by employing helically lobed rotors with backflow into the transfer volumes provided by expanding orifices integral with the outlet port and disposed substantially midway between the ends of the helical lobes. This arrangement decreases the distance backflow air has to travel between the adjacent lobes of each transfer volume and increases the time or number of rotational degrees the rotor lands are in sealing relation with the cylindrical walls of the rotor chambers.